X-ray systems are commonly used in medical applications to provide intrusive-free visual assessment of tissues and bones. Such systems are commonly found in hospitals, clinics and even veterinarian clinics to assist doctors in diagnosing problems with their patients.
Conventional X-ray systems use films exposed to an intensifying screen (also known as a phosphorous screen or fluorescent screen). This screen converts the high-energy photons (X-rays) that reach the screen into lower energy photons (typically visible light), which then imprints the object being exposed onto a film. The film is then developed and subsequently viewable with a suitable backlighting source, such as a light table for example. This process is well known and has been refined for over a century, and is capable of providing high-resolution images.
A conventional table X-ray system is shown in FIG. 1. This particular type of x-ray system is commonly used in veterinarian clinics. X-ray system 10 includes a table 12 with a working surface 14 made of bakelite or formica. Mounted on the underside of the working surface 14 is a detachable film cartridge/cassette 16, which is physically connected to an x-ray emitter 18 via support arms 20. The film cartridge/cassette 16 includes an intensifying screen in proximity to the underside of the working surface 14, and film underneath the screen for capturing the photons emitted from the intensifying screen. The x-ray emitter 18 is fitted with an x-ray tube 22 and a collimator to help confine emitted x-rays to the area of interest. The x-ray emitter 18 and film cartridge/cassette 16 are configured such that x-rays from x-ray emitter 18 will always reach the film cartridge/cassette 16. In the presently shown example, film cartridge/cassette 16, x-ray emitter 18 and support arms 20 are slidable along the working surface 14 as a unit in the directions indicated by the arrows in FIG. 1. This allows the operator to position the emitter/cartridge assembly as desired. A generator 24 typically mounted to a nearby wall or under the table 12 supplies power to x-ray emitter 18. An alternate configuration has the intensifying screen mounted vertically, with the emitter/cartridge assembly slidable in the vertical axis, which is more suitable for chiropractors, for example. Those of skill in the art will understand that working surface 14 can be made of any suitable material that facilitates the transmission of X-rays to the intensifying screen of the cartridge/cassette 16.
FIG. 2 shows further details of the cartridge/cassette 16 shown in FIG. 1, and in particular, the layered composition of the cartridge/cassette 16. As previously shown, cartridge/cassette 16 is positioned underneath working surface 14 of the table 12. The area of cartridge/cassette 16 is approximately 14×17 inches, with a thickness that is determined by the thickness of the individual layer components. The cartridge/cassette 16 generally includes an optional X-ray grid 25, an auto ionization chamber 26, an intensifying screen 28, and film 30. As illustrated in FIG. 2, X-rays 32 pass through an object (not shown) placed on top of the working surface 14, through the X-ray grid 25 and ionization chamber 26. The X-rays colliding with the intensifying screen 28 cause light to be emitted towards the film 30 for image capture. X-ray grid 25 functions as a filter to prevent scattered X-rays 34 deflected from the object from passing through to the intensifying screen 28, as these scattered X-rays contribute to image noise, and are hence undesirable. Ionization chamber 26 is a device that intercepts a portion of the X-rays and sends a signal to the operator when a predetermined dose has been reached. The ionization chamber 26 includes collection zones of a predetermined area that would be positioned beneath vital organs of a patient, such as lungs, to ensure that sufficient X-rays are received to obtain a satisfactory image. Unfortunately, the ionization chamber 26 effectively blocks some of the X-rays 32 from reaching the intensifying screen 28, thus requiring an increase in the emitted dose to compensate for the absorption by the ionization chamber 26. Thus, the loss of X-rays reduces the efficiency of the system.
In operation, the operator places the specimen upon the table and positions the emitter/cartridge assembly. Activation of the x-ray emitter 18 at a particular energy level (kV), current (mA), and time in seconds (s), determines the delivered dose. Once the desired dose has been delivered, the film cartridge/cassette 16 is detached from the assembly and developed in a dark room.
The disadvantages of film-based x-ray systems include film cost, chemical developer cost, exclusive use of a room as a dark room (or the purchase of an automatic film processor), and single master copy of the image. Of course, this leads to further disadvantages such as storage for the film and chemicals, proper disposal for used film and chemicals, and careful packaging and mailing of the single image copy to other experts when further assessment is required.
Naturally, with the advent of digital imaging technology and charge coupled device (CCD) technology, filmless X-ray systems have been developed to directly take X-ray images and display them on a computer screen for immediate evaluation. Although the digital imaging process is significantly faster and solves many of the disadvantages inherent with film based systems, presently available digital filmless x-ray systems provide image quality inferior to those of film-based x-ray systems or high-quality systems at a prohibitive cost for private clinics.
A critical factor to image quality in digital x-ray systems is the ability of the CCD sensor to collect sufficient light emitted by the intensifying screen. X-ray films are positioned directly underneath the intensifying screen, and thus maximize emitted light collection. A digital sensor on the other hand, must use an optical element that redirects the light from the intensifying screen onto the CCD. In order to collect the complete image from the 14×17 inch intensifying screen, the CCD must be a certain distance apart from the screen. The distance is significant due to the large viewing angle of the optical design which redirects a 14×17 surface to the tiny surface of the CCD. Consequently, in most optical designs, only a small portion of the light is collected. For example, only about 2% of the light emitted from the intensifying screen will reach the CCD.
In addition the electronic circuits of the CCD and those coupled to the CCD can be damaged by X-rays that pass through the working surface. The CCD circuit assembly must therefore be positioned outside of the path of emitted X-rays and, depending on the positioning of the CCD circuit, redirection of the emitted photons is required.
This usually leads to increased size of the CCD sensor and associated optics assembly to facilitate the optical design and improve system performance, hence increased system costs and size. Ideally, the housing of the CCD sensor and its associated optics should be minimized so as not to overly limit the range of positioning of the X-ray emitter and CCD sensor assembly. For example, in the veterinary clinics, the X-ray assembly must be positionable anywhere along the working surface. In a chiropractor clinic, patients typically stand, therefore the X-ray assembly must be positionable anywhere vertically.
Many designs have been proposed to redirect the light path and increase the amount of light being collected. The most common technique includes combinations of spherical lenses that collect light over a large surface and concentrate the light beams onto one CCD chip. Unfortunately, to collect as much light as possible, it is imperative to position the lens assembly as close to the screen as possible, or use very large lenses. The resulting image distortions are significant and limit the usefulness of this technique. Furthermore, the lens design prevents the light from being evenly distributed, and more light is collected at the center of the screen than at the edges.
Another common technique is to use many CCDs. Since the area to cover per CCD is smaller, the lens elements can be put closer to the screen, thus collecting more light and increasing the overall system resolution. The resulting image is a combination of images generated by each CCD and merged/tiled together. In order to realign pictures, each CCD slightly overlaps its neighbouring CCD. However any distortion in the source images will increase the complexity of aligning the images together. Complex DSP programs and CCD alignment procedures are therefore required to minimize alignment problems, but undesired artifacts may be created. This, of course, has also the disadvantage of increasing the system cost.
Another limitation of prior art digital X-ray imaging systems is the inconsistent image quality over the intensifying screen. As the CCD is brought closer to the intensifying screen, thus increasing the viewing angle, image sharpness is lost since the light energy is dispersed over several pixels, especially at the edges of the image. Therefore, inconsistent quality across the image is obtained, even though relatively large amounts of light may be collected. This effect limits quality image capture to a smaller area of the intensifying screen.
It is, therefore, desirable to provide a digital filmless x-ray imaging system which can maximize collection of emitted light from an intensifying screen while minimizing distortions and other image degrading effects.